In response to market trends and government regulations, the Pulp and Paper Industry throughout the world is presently experiencing a transition from chlorine-based bleaching methods to non-chlorine or reduced-chlorine-based bleaching methods. The technical literature is presently replete with technical advancements extolling the performance of Totally Chlorine Free (TCF) bleaching processes or Elemental Chlorine Free (ECF) bleaching processes. The former processes employ no chlorine-containing chemicals at all, while the latter processes use chlorine dioxide as the only chlorine containing bleaching chemical along with other non-chlorine bleaching agents.
This trend toward eliminating or minimizing the presence of chlorine-containing compounds from the bleaching process also introduces the potential of re-using the non-chlorine containing liquid streams generated in the bleach plant of a pulp mill. In the trade and technical literature such xe2x80x9cclosedxe2x80x9d mills are now described by the expressions Effluent Free Mill (EFM) or Closed-Cycle Mill. In the past, the collection and re-use of these liquids was uneconomical or technically impossible due to the potential corrosion damage or to interference with the process chemistry caused by the chlorine-containing chemicals in the systems and equipment used to recover and re-use these liquids. These chlorine-containing effluents were typically xe2x80x9cseweredxe2x80x9d and then, with or without treatment, discharged to the local xe2x80x9crecipientxe2x80x9d, that is, to a lake, river or other body of water However, as disclosed in U.S. Pat. Nos. 5,374,333; 5,300,19, 5,302,246, 5,439,555 and copending application Ser. No. 08/113,642 filed on Aug. 31, 1993, now U.S. Pat. No. 5,549,788 and marketed under the trademark MIM by Ahlstrom Machinery of Glens Falls, N.Y., several processes and systems have been developed for reducing or eliminating chlorine-containing compounds in pulp mill liquid streams and effluent streams, or for recovering and re-using these valuable chemical-containing resources with no or minimal effect upon the surrounding environment.
However, current examination of the process chemistry, either theoretical or in mill trials, has revealed that this reduction or elimination of chlorine-based bleaching compounds is hindered by the presence of metal ions in the process streams. These metal ions, for example, ions of iron, copper, manganese, and magnesium, among others, interfere with process chemistry, for example pulping and bleaching, and can accumulate in the pulp mill and become manifest as precipitation deposits, that is xe2x80x9cscalexe2x80x9d, on equipment.
These metals can be introduced to the pulp mill from several sources. For example, the wood supply may contain metal ions that naturally exist in the trees or other fiber source used. Metals may also be introduced as impurities in the chemicals introduced to the pulp mill, for example, in purchased acids or make-up chemicals. Metals ion may also originate from corrosion in the equipments itself.
In the past, when chlorine-based bleaching processes were the norm, the presence of metal ions in the pulp or process streams was not significant. For example, chlorine in an acidic environment solubilized the metal ions and they were subsequently removed from the system with the chlorine bleach effluent. As a result, the metal ions were simply purged from the system when the chlorine-containing bleach effluent was sewered. However, present non-chlorine-based bleaching chemicals, such as ozone and peroxide, do not react with the metal ions in a beneficial fashion but tend to be consumed by metal ions and thus these ions negatively effect the bleaching reaction. It is now believed that in addition to consuming, for example, peroxide, metal ions may catalyze reactions which produce by-products that undesirably also consume bleaching chemical. Thus the presence of dissolved metal ions in the bleaching stages reduces the efficiency, and hence increases the cost, of the bleaching process. To address this problem, typical conventional TCF or ECF bleaching sequences incorporate some form of metal-removing treatment, for example, a treatment with chelating agents (also known as sequestering agents or metal-complex forming agents) or an acid wash, or a xe2x80x9chot acidxe2x80x9d treatment as disclosed in pending, U.S. application Ser. No. 08/542,646 filed on Oct. 13, 1995, and now abandoned.
In this specification and claims the term xe2x80x9cchelating agentxe2x80x9d or xe2x80x9cchelantxe2x80x9d is used to refer to any chemical compound having strong affinity for transition metal ions, including Mn, Fe and Cu ions, and tending to combine with the metal ions. This process is also referred to the xe2x80x9csequesteringxe2x80x9d of metal ions; thus these compounds are also referred to as xe2x80x9csequestering agentsxe2x80x9d. Furthermore, the chelant-metal ion compounds that are formed are often referred to as xe2x80x9ccomplexesxe2x80x9d; thus chelating agents are also referred to as xe2x80x9cmetal complexing agentsxe2x80x9d or xe2x80x9ccomplex forming agentsxe2x80x9d. The term xe2x80x9cchelating agentxe2x80x9d as used herein encompasses all these terms, and the released transition metals combined with chelating agent are referred to as xe2x80x9ccomplexesxe2x80x9d.
The presence of dissolved transition metal ions also hampers the process of mill xe2x80x9cclosurexe2x80x9d, that is, the recovering and re-using effluents. As discussed above, the presence of metals in a bleach plant is typically minimized by some form of metal-removal treatment. However, if the effluents resulting from such treatment, which contain dissolved metal ions, are to be reused the metal ions must be removed. U.S. Pat. No. 5,401,362 and co-pending applications Ser. No. 08/113,645 filed on Aug. 31, 1993 and Ser. No. 08/195,139 filed on Feb. 14, 1994 illustrate several methods of treating metal-containing effluents to remove the metals prior to re-use.
In addition to these treatments of the effluent streams, the presence of transition metal ions within the pulp mill can also be minimized by purifying the chemicals introduced to the mill to eliminate their introduction of metals. Also, non-corrosive metallurgy can also be used to minimize or eliminate the potential for introducing dissolved metals from corrosion. However, regardless of these and other remedies for reducing the introduction of metal ions to the pulp mill, metal ions can still enter the mill with the original wood supply, or other source of cellulose.
Published PCT application WO 95/02726 discloses one method that attempts to reduce the metal ion concentration prior to digestion in a digester. The disclosed process includes a treatment of cellulose material, for example, softwood chips, with a liquid containing a chelating agent prior to formal digestion of the material. Though this treatment reduces the concentration of various dissolved metals, it does not treat the material with chelating agents in the most advantageous stage. Since metals-containing substances are released from the cellulose in essentially all phases of the cook, treating and removing the metals prior to the cook as disclosed in the PCT application is not the most advantageous treatment. There are certain stages of the cooking process where more metal ions are released from the cellulose and can be more effectively removed. Furthermore, some metals are inherently removed during the kraft cooking process. Chelating chemicals need not be wasted removing metals that would be removed from the process anyway. In addition, the PCT publication does not recognize other significant features of the present invention.
Though the chemical mechanism is not yet completely understood, it is believed that the naturally-occurring metal ions that are present in cellulose material , for example, wood chips, are strongly bound to the wood material. As shown in the article xe2x80x9cThe Behavior of Certain Inorganic in the Wood/White Liquor Systemxe2x80x9d by Hartler, et al. (Svensk Papperstidning, No. 12, 1973), these metal ions are typically not released from the cellulose until well into the pulping process, for example, not until the pulping temperature reaches at least 50xc2x0 C., preferably at least about 100xc2x0 C., or even about 150xc2x0 C. or more, Only at these temperatures or higher will the metal ions present in the cellulose be released into the slurry liquid; only at these temperatures will these ions be available for sequestering by a chelating agent or re-attachment to the cellulose material.
In addition to chelating agents, other naturally occurring compounds, such as lignin and carboxylic groups in the carbohydrates, will also compete for the released metal ions. Since it is undesirable for the released metal ions to become attached or re-attached to these organic compounds and be retained in the pulp stream, the effectiveness of the metal removal treatment is dependent upon the addition point of the chelant. If the chelant is present at a point where the lignin concentration or carboxylic group concentration is high, the metal removal may not be as effective as if the chelant were present when the concentration of these compounds is lower. Therefore, a preferred location for having chelating agents present in the cooking process is at a point where the temperature is above about 100xc2x0 C., typically above about 130xc2x0 C., and preferably above about 150xc2x0 C., and where the dissolved organic material [including lignin, hemi-cellulose, and other organics as disclosed in U.S. Pat. No. 5,489,363] concentration is low, typically, less than about 120 g/l, preferably, less than about 100 g/l, and even less than 90 g/l.
Another consideration when attempting to minimize the presence of dissolved metal ions during chemical pulping is the presence of hexeneuronic acids. As recently discovered, compounds generally referred to as hexeneuronic acids are formed during alkaline pulping. These compounds are not naturally-occurring in the cellulose, for example, wood chips, but are formed during the alkaline pulping process. For example, naturally-occurring 4-0-methylgluconic acid (MeGlcA) transforms into hexeneuronic acid (HexA) during the kraft cooking process. These electrically charged hexeneuronic acids are one of the primary sources of charged sites on the cellulose fiber. It is believed that these charged sites provide the means by which metal ions typically are attached to cellulose fibers.
The effect of the presence of these hexeneuronic acids is illustrated by experience in bleaching pulps produced from non-alkaline processes. It has long been known that it is relatively simpler to peroxide bleach pulps produced from the acid sulfite process or by mechanical processes, both non-alkaline processes, than it is to peroxide bleach alkaline kraft pulps. However, the reason for this difference has not been Understood. It has now been realized, surprisingly, that the above difference is related to the presence of the above-mentioned hexeneuronic acids. It has been discovered that hexeneuronic acids do not exist in both sulfite and mechanical pulps. As noted above, it is known that metal ions consume peroxide or at least degrade the peroxide bleaching process. Since the hexeneuronic acid provide sites for attachment of metal ions, minimizing these acids will minimize the sites to which metal ions can attach to cellulose. Thus, minimizing the concentration of charged acid groups, for example, hexeneuronic acids, minimizes the transport of metal ions to the metal ion-sensitive bleaching stages and therefore improves the bleaching of pulp.
In its broadest concept the present invention comprises a method of treating comminuted cellulosic fibrous material during the production of chemical (e.g., kraft) pulp so that the presence of bleaching-chemical-consumability of the resulting pulp is minimized (e.g., the metal ions are minimized in the resulting pulp). One method of effecting this is to have chelating agents present in the cooking process when the metal ions are released from the cellulose.
Thus, according to one embodiment of the present invention there is provided a method of producing chemical cellulose pulp from comminuted cellulosic fibrous material containing metal ions, using a cooking vessel, comprising the steps of: (a) Steaming the comminuted fibrous material to remove air and begin heating thereof [this stage necessary only in continuous treatment, not batch]. (b) Pressurizing and slurrying the material in cooking liquor and introducing the slurry of material an cooking liquor into a cooking vessel. (c) Heating the material to at least a first temperature such that at least a significant amount of the transition metal ions are released to the slurry liquor. (d) Treating the slurry with a chelating agent which combines with at least the released transition metal ions in the slurry to produce complexes. (e) Cooking the material in the cooking vessel at a second temperature (which may be higher than said first temperature, but may be about the same or if the first temperature is high enough even lower) to produce chemical pulp; and (f) discharging the chemical pulp from the cooking vessel. The invention preferably includes the additional step (g), between steps (d) and (e) of removing at least some lo of (preferably at least a majority of) the chelated metal ions (complexes) from the slurry, and during step (c) at least 10% of the transition metal ions are released.
The treatment with chelating agent may be for a time period between about 5 minutes to 6 hours, but it is typically performed for between about 10 to 180 minutes, or preferably between about 15 to 120 minutes.
Typical chelating agents include EDTA and DTPA, and their equivalents, but certain organic acids may also be used, for example, oxalic acid, tartaric acids, and furoic acid. These organic acids are preferably obtained from existing pulp mill processes, for example, a bleaching process. The chelating agent is preferably one that is temperature resistant, that is, it is effective at typical cooking temperatures, about 150xc2x0 C., and can withstand high pH, that is, a pH greater than 10, preferably greater than about 12. The dosage of chelating agent ranges from 0.1 to 10 kg. per ton of pulp, and the dose is preferably between about 1-5 kg per ton of pulp. This dosage is effective to combine with at least about 10% of the metals released from the material, typically at least a majority of the released metals, and preferably substantially all (i.e. greater than 90%) of the released metals, to form complexes.
The invention is preferably performed wherein said first temperature of step c) is at least about 100xc2x0 C., typically at least about 130xc2x0 C., and preferably at least about 150xc2x0 C. The second temperature of step e) is typically between about 140 and 180xc2x0 C., preferably between about 150 and 170xc2x0 C., and at least about 160xc2x0 C. if the first temperature is about 150xc2x0 C. The cooking vessel of step b) is preferably a pretreatment vessel or impregnation vessel or digester. The treatment may be performed continuously or in a batch process.
Since some of the material dissolved and released into solution during the cooking process are the undesirable transition metal ions, removing spent cooking liquor containing dissolved transition metals from the cooking process minimizes the concentration of dissolved transition metals in the resulting pulp. The process of minimizing the concentration of dissolved material in the pulping process in general is disclosed in U.S. Pat. No. 5,489,363 (the disclosure of which is hereby incorporated by reference herein) and is marketed under the trademark LO-SOLIDS by Ahlstrom Machinery of Glens Falls, N.Y. However, another embodiment of this invention is the combination of the process of U.S. Pat. No. 5,489,363 with the addition of chelating agents. In combining these two processes, the chelating agent is introduced to the pulp when the concentration of dissolved transition metal ions is already minimized and the chelating agent neither competes with the chelating effect of the dissolved organic material nor is wasted on metal ions that are already attached or will soon be attached to dissolved organic material and removed. This embodiment of the invention thus comprises a method for producing pulp by cooking comminuted cellulosic fibrous material by: a) extracting liquor containing a level of dissolved organic material; b) replacing some or all of the extracted liquor with liquid containing a substantially lower level of effective dissolved organic material than the extracted liquor; and wherein the replacement liquor of step b) contains at least one compound that combines with transition metal ions and can subsequently be readily removed. The compound is typically a chelating agent as described above. The chelating agent may be fresh EDTA or DTPA, or may be bleach plant effluent. Some bleach plant effluents contains acid compounds, for example, furoic or maleic acids, that act as chelating agents.
In another specific application of this invention, the metal-complex former or chelating agent is preferably added downstream of a liquor extraction or after or during the process of extracting and diluting the cooking liquor as in the process of U.S. Pat. No. 5,489,363. This chelate treatment may be practiced in a co-current or counter-current cooking mode, in a single or multi-vessel digester system, in a hydraulic or dual phase digester, or in a batch or continuous fashion.
In a continuous digester, the chelating agent is preferably added downstream of a counter-current cooking zone or upstream of a co-current cooking zone. For example, the chelating agent is preferably added to the lower cooking circulation or the wash circulation of a single-vessel Lo-Solids(copyright) digester. In a batch digester, the chelating agent is preferably added when the cellulose material is heated by displacement with spent cooking liquor to between about 140 and 165xc2x0 C. or shortly thereafter.
Chelating agents may also be added to the first washing stage following the cooking stage, either within or external to the digester, and the chelating agent passed counter-currently to the latter stages of the cooking process where the dissolved solids concentration is relatively low. For example, chelating agents can be added to the xe2x80x9ccold blowxe2x80x9d circulation of a Kamyr(copyright) continuous digester, sold by Ahlstrom Machinery, or to the weak liquor used for displacement in the final stages of a batch cooking process.
In another preferred embodiment, the transition metal removing agent, for example chelating agent, may be displaced into the cellulose material after pretreatment and before the formal cook. Though, as described above, co- or counter-current displacement of the chelating agent can be performed in a continuous or batch digester, the chelating agent may also be radially displaced into the pulp. For example, after the cellulose material slurry is heated above a temperature at which metal ions are released into solution, for example, to about or above 140xc2x0 C., the chelating agent may be radially displaced through the slurry by means of cooking liquor added in a cooking circulation. For instance, after pretreatment of the cellulose material, for example with spent cooking liquor, the time required for impregnating the material with cooking liquor, for example kraft white liquor, is relatively short, typically only about 5 to 10 minutes. Subsequently, the radial displacement treatment with a chelating agent and impregnation may be performed via a single screen assembly in a continuous digester, but two screen assemblies are preferred. This horizontal radial addition of chelating agent may be performed at the bottom of a pretreatment vessel or at the top of a digester.
Another method for reducing the metal ion content of the cooked pulp is to prevent the formation of electrically charged sites on the pulp, for example, charged sites due to charged acid groups, specifically, hexeneuronic acids, to which metal ions are drawn. This can be achieved by increasing the alkalinity of the cooking liquor. This increased alkalinity neutralizes the acid groups such that charged metal ions do not become attached to the pulp. The alkalinity of cooking liquor is typically expressed as grams per liter of effective alkali (EA), expressed as NaOH [that is, g/l EA as NaOH]. In conventional alkaline cooking, this EA is consumed during the cooking process such that at the end of the cook the EA is about 10 g/l or less. By increasing the alkalinity during or at the end of the cook and effectively raising this EA at the end of the cook, for example to at least about 15 g/l (e.g., 15-40 g/l), preferably between about 18-25 g/l expressed as sodium hydroxide, the charged acid groups are preferably neutralized and their charged sites reduced. For example, charged hexeneuronic acids can be reduced by at least about 30%, typically at least by about 50%, or by as much as about 80% by raising the alkalinity at the end of the cook. As a result, transition metals ions do not attach to the cellulose and are not retained in the pulp.
For example, according to another exemplary method of producing chemical cellulose pulp according to the invention, the following steps are practiced: (a) Cooking comminuted cellulosic fibrous material at a temperature of between about 140-180xc2x0 C., and at an effective alkalinity such that the charged acid group concentration during cooking is at least about 30% less than the concentration of charged acid groups present after cooking of the same material at an effective alkalinity of about 10 g/l or less, expressed as NaOH, to produce a chemical pulp; and (b) bleaching the chemical pulp in at least one non-chlorine bleach stage (any one or more conventional non-chlorine bleaching stages or sequences can be used, including those with oxygen (including oxygen delignification), ozone, peroxide, hydrosulfite, etc.). Preferably during at least the majority thereof step (a) is practiced at an effective alkalinity of between about 15-40 g/l (most desirably 18-25 g/l) expressed as NaOH. The method may also comprise the further steps of: (c) heating the material prior to step (a) to a temperature of at least about 100xc2x0 C. (e.g., at least about 130xc2x0, preferably at least about 150xc2x0 C.) so as to release at least 10% (and preferably at least a majority of) the transition metal ions therein; (d) adding chelating agent in the amount of 0.1-about 10 (preferably about 1-5) kg per ton of pulp so that the chelating agent is present during the practice of step (a), and so that it combines with at least about 10% (and preferably at least the majority) of the released transition metals to form complexes; and (e) removing at least 10% (and preferably at least the majority) of the complexes prior to step (b). During treatment with chelating agent the slurry preferably has a dissolved organic material concentration of less than about 120 g/l (preferably less than 100 g/l, and even less than about 90 g/l).
According to another aspect of the present invention a method of producing chemical cellulose pulp is provided comprising the following steps: (a) Treating comminuted cellulosic fibrous material having metals therein, slurried with cooking liquor, to a temperature of at least about 100xc2x0 C. to a significant amount of the transition metals therefrom, and so that the slurry has a dissolved organic material concentration of less than about 120 g/l. (b) Treating the slurry from step (a) with a chelating agent in an amount effective to combine with at least a significant amount of the released transition metals to produce complexes. (c) Cooking the slurry from step (b) to produce a low non-chlorine bleaching chemical consuming content chemical pulp; and (d) bleaching the chemical pulp from step (c) in at least one non-chlorine bleaching stage (as described above, i.e. any non-chlorine bleaching stage or sequences).
In the method described above, typically step (a) is practiced at a temperature of at least about 130xc2x0 C., and to release at least about 10%, preferably at least about 50%, of the transition metals from the material, and so that the dissolved organic material concentration is less than about 100 g/l; and step (b) is practiced by treating the slurry with a chelating agent in an amount effective to combine with at least about 10%, and preferably at least about 50%, of the released transition metals (e.g., between about 1-5 kg per ton of pulp produced). There preferably is also the further step (e), between steps (b) and (d), of removing a majority of the complexes, such as by using extraction screens in a continuous digester. Step (b) may be practiced by adding bleach plant effluent. Step (c) may be practiced at a temperature greater than the temperature of step (b), and between about 140-180xc2x0 C.; and step (e) may be practiced between steps (b) and (c). Alternatively, step (b) may be practiced at a temperature of at least about 150xc2x0 C. and step (c) at a temperature of at least about 160xc2x0 C. Step (b) preferably is practiced at a pH of at least 10, and preferably at least about 12. Also, at least the majority of step (c) is practiced at an effective alkalinity of between about 15-40 (preferably about 18-25) g/l expressed as NaOH, so as to reduce the concentration of charged acid groups, including hexeneuronic acids, in the pulp produced in step (c), by at least 30% compared to if step (c) is practiced at an effective alkalinity of less than about 10 g/l expressed as NaOH.
The invention also relates to a new pulp, having lower non-chlorine bleaching chemical consumability than conventional kraft pulp. For example according to another aspect of the invention, pre-bleached low bleaching chemical consumption kraft pulp is produced by the steps of: (a) treating comminuted cellulosic fibrous material having transition metals therein, slurried with cooking liquor, to a temperature of at least about 100xc2x0 C. to release a significant amount of the transition metals therefrom, and so that it has a dissolved organic material concentration of less than about 120 g/l; (b) treating the slurry from step (a) with a chelating agent in an amount between 0.1-10 kg/ton of pulp, and effective to combine with a significant amount of the released transition metals to produce complexes; (c) kraft cooking the slurried material from step (b); (d) preparing the pulp from step (c) for bleaching in at least one non-chlorine bleaching stage; and (e) removing a significant amount (e.g. a majority) of the complexes between steps (b) and (d) so as to produce kraft pulp having a bleaching chemical consuming metals content at least 30% lower than conventional kraft pulp produced without the practice of steps (b) and (e). This pulp may also be used to produce a paper product containing minimal amounts of transition metal-containing compounds.
It is the primary object of the present invention to produce chemical pulp which consumes less non-chlorine bleaching chemicals than conventional chemical pulps produced from the same raw material (e.g., softwood chips). This and other objects of the invention will become clear from an inspection of the detailed description of the invention, and from the appended claims.